Automatic steering mechanisms



Nov. 10, 1953 R. J. KUTZLER 2,659,041

AUTOMATIC STEERING MECHANISMS Filed Aug. 25, 1952 VERTI CAL GYROS COPE up T jELEVATOR INVENi'OR. ,ROBERT J. KUTZLER ATTORA/EX Patented Nov. 10, 1953 UNITED STATES PATENT OFFICE AUTOMATIC STEERING MECHANISMS Robert J. Kutzler, St. Louis Park, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application August 25, 1952, Serial No. 306,189

12 Claims. 1

This invention pertains to automatic steering mechanisms for dirigible craft such as aircraft. Such steering mechanisms may include a type of servomotor that may be operatively clutched with or engaged with a control surface. Such control surface may also be directly manually positioned. During this direct manual positioning of the control surface, the servomotor is operatively disengaged or declutched from the control surface.

The disengagement of the servomotor from its control surface may be selectively manually effected as a preliminary step to the assumption of direct manual control of the surface. However, there are occasions when automatic disengagement of a servomotor from its control surface is desirable. As an illustration, consider the pitch channel of an automatic steering mechanism wherein the elevator surface is operated by the elevator servomotor. The disengagement of this elevator servomotor from its surface may be desirable in one situation where due to an up or down draft the linear acceleration of the aircraft is such that the permissible stress on the craft structure is being approached and it is desirable to avoid overstressing the craft. If the servomotor be automatically disengaged from its control surface before the craft structure is overstressed, the elevator control surface may be directly manually operated to reduce the acceleration. 1

While automatic disengagement of the servomotor and its control surface at an acceleration slightly less than that acceleration which would cause overstressing the craft is normally an acceptable arrangement where the acceleration is due alone to an external disturbance on the craft such as the aforementioned up or down draft, which did not involve as an incident to the application of the stress the operation of the elevator control surface, it is desirable to reduce the magnitude of the acceleration at which disengagement occurs during actual operation of the surface by the automatic steering mechanism. This is particularly true where the direction of operation of the control surface is such that it tends to increase the acceleration of the craft above that occasioned by the external disturbance.

Other situations may arise where it is desirable to lower the magnitude of the acceleration at which disengagement occurs. One situation is contemplated where the servomotor is used in a balanceable or follow-up system that controls the extent of servomotor operation and wherein the elevator control surface displacement in the follow-up system is proportional to a change in craft pitch attitude as detected by a sensing device in the balanceable system. If the servomotor continued to displace the elevator surface, due to a malfunction of the balanceable system preventing its rebalance, the angle of attack of the aircraft would be increasing. Increase in angle of attack, within limits, causes an increase in lift. An increase in lift in turn increases upward or linear acceleration of the craft. As acceleration increases, disengagement of the servomotor and surface to prevent overstress may result.

However, the aircraft has an angular momentum about its pitch axis due to the continuously operated elevator surface; and even when disengagement of the servomotor and surface occurs, the angular momentum of the aircraft about its pitch axis due to the displaced elevator surface will cause an increase in the angle of attack beyond that at disengagement. It is this increase in the angle of attack that results in a higher linear acceleration beyond that at which disengagement normally occurs which might cause overstressing of the craft structure.

It is therefore an object of this invention to provide a novel arrangement in an aircraft for operatively disengaging a control surface of the craft from its servomotor.

It is a further object of this invention to provide a novel arrangement in an aircraft for operatively disengaging a control surface of the craft from its operating servomotor when the linear acceleration of the craft in a direction normal to its pitch and roll axes attains a predetermined value.

It is a further object of this invention to provide a novel arrangement in an aircraft for op eratively disengaging a control surface of the craft from its operating servomotor when the craft attains a predetermined magnitude of linear acceleration and to automatically reduce the magnitude of the acceleration at which such disengagement occurs in one direction when the control surface is operated by an automatic steering mechanism of the craft.

It is a further object of this invention to provide an aircraft with an automatic steering mechanism for positioning the elevator control surface of the craft through a servomotor operatively engaged with the surface and wherein disengagement of the surface from its servomotor occurs when the craft attains a linear acceleration in either direction along a line normal to the roll and pitch axes of the aircraft and wherein the value of acceleration in one direction in which automatic disengagement occurs is decreased upon the positioning of the elevator surface from the servomotor.

It is a further object of this invention to provide a steering mechanism for an aircraft wherein a servomotor is operatively engaged with a con trol surface of the craft and wherein the servomotor is operatively disengaged from the surface automatically when the craft attains a predetermined value of linear acceleration and wherein further the magnitude of the acceleration which causes the disengagement of the servomotor and control surface is decreased when the servomotor operates the control surface in a direction tending to increase the acceleration of the craft above that occasioned by an external disturbance acting on the craft.

The above and further objects of the invention will become apparent hereinafter upon consideration of the following description of a preferred embodiment of the invention taken in conjunction with the annexed drawing illustrating the same.

The drawing is a schematic representation of a servomotor is of the reversible D. C. type and comtrol surface of a conventional aircraft through a clutching arrangement responsive to a novel craft acceleration monitoring device.

Referring to the drawing, the elevator control surface (not shown) of the aircraft is operated by main control cables ll] extending from a conventional manual controller H. The elevator surface may also be operated by power operated cables |2 extending from a cable drum |3 which cables pass over guide pulleys l4, l5, and are suitably connected to the main cables whereby motion of the drum I3 is communicated to the main cables. The cable drum I3 is carried on an output shaft l6 of an elevator servomotor IT. The servomotor isof the reversible D. C. type and comprises field windings |8, |9, armature 2i), combination pulsing clutch brake 2|, and magnetic clutch 22 for operatively engaging the servomotor I! with the cable drum I3. The servomotor I1 is connected through an operated elevator engage relay 53 to an amplifier 52 which reversibly controls the rotation of the servomotor l1 and is also connected through the relay to a source of voltage such as a battery 41 for operatively connccting or clutching the servomotor ll with the elevator control surface. The amplifier 52 is of the A. C. discriminator type and its operation is controlled by a balanceable control signal network 50. The continued operation of the elevator engage relay l8 and thus the continued operative engagement of the servomotor H with the cable drum l3 and thus with the elevator control surface is controlled by an accelerometer device I03 which in response to vertical acceleration of the aircraft in either upward or downward direc tion above a predetermined magnitude interrupts a circuit of winding which effects the operation of relay l8 and thus disengages the servomotor I! from its control surface.

An understanding of the details of the invention may be obtained from the following complete description. The servomotor field windings l8 and I9 are connected together to have one common junction. Extending from this common junction and connected in series in the following order are conductor 23, motor armature 20, conductor 24, pulsing clutch brake 2|, conductor 25, which is connected to ground. The relay 53 is integral therewith.

4 of the four pole single throw single make type comprising an operating winding 35, relay arms 36, 31, 3'6, and 39 which engage respectively with in contacts 40, 4|, 42, and 43. The free end of motor winding I8 is connected by conductor 21, accelerometer control winding IE, to be described, conductor 29 to relay in contact 4|. The free end of motor winding I9 is connected to conductor 30, accelerometer control winding I I5, conductor 26 to relay in contact 43. The input side of magnetic clutch 22 is connected by conductor 3| to in contact 42 of relay l8 and the output side of the clutch is connected through conductor 32 to ground. The remaining in contact 40 of the relay l8 constitutes part of a holding circuit to be described for maintaining the relay energized. Relay arm 36 is connected by conductors 45, 46, to a source of D. C. voltage such as battery 4'1. Relay arm 31 is connected by conductor 48 to the output side of elevator amplifier 52. Relay arm 38 is connected by conductor 49, 50, to battery 41. Relay arm 39 is connected by conductor 5| to the output side of elevator amplifier 52.

The amplifier 52 which is of the A. C. discriminator type has A. C. power input connections 54, 55 extending to the ships supply and alternating voltage signal input connections 55, 5! connected to the balanceable network 60. The amplifier includes a pair of alternatively energized single make relays 58, 59. One or the other of the relays in the amplifier is energized depending upon the phase relationship of the voltage across signal input connections 56, 51 relative to the voltage across the power input connections 54, 55. The amplifier 52 may be of the type disclosed in Pat ent 2,425,734 to Willis H. Gille et a1. Such amplifier is so constructed as to have an on time and an off time for the amplifier relays. For large values of input control voltage the amplifiers are continuously closed but for smaller values of control signal voltage the relays are intermittently operated thereby to decrease the on time. As the control signals become smaller, the on time decreases until when there is no control signal supplied the relays remain unenergized.

Control voltage signals are obtained from the balanceable network 65 comprising series connected voltage generators BI, 62, 63, and B4. Signal generator 5| comprises a rebalancing potentiometer 61 having a slider 63 and a resistor 6?) which is connected across a secondary winding 13 of a transformer 12 having a primary winding 74. Slider es is positioned along resist-or 69 through a suitable follow-up operating connection '55 extending from the output shaft it of servomotor ll. Amplifier conductor 58 extends to slider 58. Signal generator 62 comprises a pitch attitude potentiometer l? having a slider 18 and a resistor 19 which is connected across a secondary winding of the transformer. Through a suitable operating connection 8|, a vertical gyroscope 82 positions slider 18 relative to resistor El in accordance with the pitch attitude of the aircraft. A conductor 83 connects a center tap of secondary winding 13 with slider i8. As indicated in signal generators 6| and 62, a single primary winding 14 of a transformer 12 is utilized to supply a plurality of secondary windings. Signal generator 63 comprises a manually operable trim potentiometer 85 having a slider 86 and resistor 81 which is connected across a secondary winding 88 of the transformer. Slider 3B is manually positioned along resistor 87 through an operable knob 89 A conductor 90 connects a.

center tap of secondary winding 89 and slider 86. Signal generator 64 comprises an altitude control potentiometer 92 having a slider 93 and resistor 94 which is connected across a secondary winding 95 of the transformer. An altitude controller 91 through a suitable operating connection 96 positions slider 93 relative to resistor 94 in accordance with changes in altitude of the craft from a preselected altitude. A conductor 98 connects a center tap of secondary winding 88 and slider 93. A conductor 99 extends from a center tap of secondary winding 95 to ground and the input circuit is completed through ground to amplifier conductor 51.

The accelerometer I99 comprises a vertically positioned support member I9I having laterally extending spring arms I92, I93 which at their free ends support weights I94, I95 of magnetic material. On the opposite side of the spring arms I92 and I93 from that to which the weights I94 and I95 are secured are carried switch contacts I96, I98 which respectively coact with switch contacts I91, I99 fastened to the support member NH. The accelerometer I99 is mounted approximately at the center of gravity of the aircraft so that it will not be responsive to angular accelerations of the craft. When the craft has a vertical acceleration normal to the pitch and roll axes of the craft the inertia of weight I95 has a tendency to cause a separation of contacts I98 and I99. Similarly when the aircraft has a downward Vertical acceleration the inertia of weight I94 tends to separate contacts I96 and I91. The magnitude of the acceleration attained in a downward direction before switch contacts I96 and I91 will be separated may be selectively varied by a spring H9 having one end connected to spring arm I92 and its other end connected to a manually adjustable screw I II passing through a flange on member I9I for varying the tension of spring I I9. Similarly for varying the magnitude of the vertical upward acceleration that would cause the separation of switch contacts I98 and I99 is a spring II 2 extending from spring arm I93 and connected to a manually adjustable screw I I3 passing through a flange on member I9I for varying the tension in spring II2. Associated with the weight I94 is a control winding or coil |I5 which when energized exerts a force on weight I94 tending to separate contacts I96 and I91. Similarly associated with weight I95 is a coil H8 which when energized exerts a force on weight I95 tending to separate contacts I99 and I99. The coils H5 and II 6 are so designed that it will take a little time for the energization of the coils to build up to their maximum strength. The coils H5 and I I5 are series with the field windings of the servomotor I1 such that during the intermittent operation of the amplifier relays 58 or 59 the energization of the coils H5 and H6 does not build up to their maximum value and have no efiect on weights I94, I95.

However when the amplifier relays 58 or 59 are continuously operated, the coils H5 and H6 will build up to their maximum attractive force and will exert a force on coils II5, IIB. Such coils are referred to as slugged coils in the art and are therefore not new herein.

Having described the operative components of the arrangement their interrelationship will become more apparent from a description of the operation of the arrangement. The aircraft in which the arrangement is installed may have its elevator control surface directly manually positioned from the controller II to attain a desired attitude in pitch while in flight. With the craft in the desired selected attitude, the trim knob 89 may be manually adjusted to balance the network 59 so that no control voltage is applied to the amplifier 52 for this attitude.

Automatic elevator operation for stabilizing the craft in the selected attitude may be initiated by manually closing the normally openmomentarily closed push button switch I29 the operation of which completes a circuit from battery 41, conductor 56, switch I29, conductor I2I, engage relay winding 35, conductor I22, to ground and return to battery ground I23. The energization of winding 35 causes operation of the relay arms 36, 31, 38 and 39. A relay operation maintaining or holding circuit is now completed from energized conductor 46, conductor 45, relay arm 36, in contact 49, conductor I24, a normally closed-momentarily opened manually operable disconnect switch I25, conductor I26, switch contact I99, switch contact I98, spring arm I93, conductor I21, spring arm I92, switch contacts I93, I 91, conductor I28, relay winding 35, conductor I22, to ground and return to battery ground I23. The engagement of relay arm 38 within contact 42 completes a circuit from battery 61, conductor 59, conductor 49, relay arm 38, in contact 42, conductor 3I, magnetic clutch 22, conductor 32, to ground and return to battery ground conductor I23. Upon energization, the magnetic clutch 22 operatively connects the armature 29 with the cable drum I3 and thus to the elevator control surface.

With the servomotor I1 engaged with its control surface, the aircraft is maintained in the selected attitude by means of vertical gyroscope 82. Should the craft change pitch attitude due to a wind gust or smaller external disturbance, the vertical gyroscope 82 adjusts slider 18 relative to resistor 19 to unbalance the network 69. This unbalance of network 69 applies a control signal to amplifier 52 to in turn cause operation of one or the other of the amplifier relays 59, 59. If relay 58 should be operated due to this unbalance of network 69, a circuit is completed from battery 41, conductor 59, relay 58, conductor 48, relay arm 31, in contact 4|, conductor 29, accelerometer coil I I9, conductor 21, servomotor winding I3, conductor 23, servomotor armature 26, conductor 24, pulsing clutch-brake 2i, conductor 25, to ground and return to battery ground I23. The energization of the pulsing clutch 2i causes it to move from its braked position to clutch position and energization of the field Winding I8 and armature 29 causing the rotation of armature 29 to be transmitted to shaft I6 in a manner similar to that in Lear 2,267,114. The operation of shaft I6 is applied to cable drum I3 to position the elevator and also through the operating means 15 adjusts slider 58 of the rebalance potentiometer 61 until the network 69 is again in balanced position. As the signal voltage from network 69 decreases the relay 58 due to the design of amplifier 52 will be intermittently closed rather than being continuously closed as when the control signals in network 69 were large. The winding I6 of servomotor I1 is thus intermittently energized as are other elements in the circuit. During the time when clutch 2| is not energized, it moves to its brake position so that the cable drum I3 is held as positioned by the ser vomotor. The accelerometer coil II6 also being .in this circuit is only intermittently energized and due to its design does not exert any appreward pull on weight I05. weight I05 decreases the magnitude of the *accelciable force on weight I05. The operation of servomotor I! has positioned the elevator control surface to restore the craft to its original attitude.

The operation of the accelerometer I will be considered when the craft is subject to a vertical acceleration due either to an up or a down draft while in flight. If the craft in its selected attitude, enters an up draft, the weight I tends to disengage contacts I08 and I09. The tension in spring II2 will have been so adjusted by the screw II3 that weight I05 will separate contacts I00 and I09 without energization of coil H6 at a magnitude of upward accelerations which is lower than that which the structure of the aircraft can withstand. Thus before the aircraft will have been overstre'ssed, the acceleration in an upward direction causes the separation of contacts I08 and I09. This separation of contacts I00 and I09 interrupts the holding circuit for maintaining winding 35 of engage relay I8 energized so that the relay arms drop to the out position. With the magnetic clutch 22 deener- .gized, the pilot may directly manually operate the controller I I to put in down elevator immediately to decrease the upward acceleration.

Similarly, if the vertical accelerations were in a downward direction the weight 104, due to its inertia, will separate contacts I06 and I0! when the predetermined magnitude of the downward acceleration has attained a value which is less than the acceleration in a downward direction which would overstress the aircraft. The above operation of the accelerometer I00 has been based on the circumstance that the automatic pilot was not involved in any controlling operation during this up or down draft.

The operation of the accelerometer will now be considered when the up or down draft is ap plied and with the automatic pilot also operating to exert a controlling effect. Should the craft infiight encounter an up draft that not only upwardly accelerates the craft but also causes it to tilt downwardly about its pitch axis, the vertical gyroscope 82 operates in response to the change in tilt to move slider 18. The movement of slider 18 unbalances network 60 and causes amplifier 52 to close relay 58 thereby energizing accelerometer coil I I8 and servomotor winding IS. The elevator control surface is driven in an upward direction by operation of servomotor I8. It will be evident that the upward positioning of the elevator control surface adds an acceleration in an upwardly direction to that applied to the craft due to the up draft alone. son of the fact that the up elevator tilts the craft in an upwardly direction about its pitch axis and thereby increases the angle of attack. This increase in the angle of attack increases the lift of the craft and therefore increases its upward acceleration.

The energization of winding I8 has also energized accelerometer coil I I6 which exerts a down This .pull on magnetic eration which the aircraft may attain before switch contacts I08 and I09 are separated. The separation of contacts I08 and I09 operatively 'disen'gages the servomotor I1 and the elevator control surface. Although the aircraft has a momentum about its pitch axis due to the displaced elevator surface at the time that the dis engagement occurs, this angular momentum while it increases the angle of attack additional ly and thus 'may cause an increase in the linear This is by reaacceleration does not overstress the craft because this, increase in acceleration above the value of the lowered acceleration at which contacts I08 and I09 were separated does not reach a total acceleration of sufficient magnitude in an upward direction that would overstress the craft structure.

Similarly if a down draft were applied to the craft and resulted also in upward tilt of the craft about its pitch axis the resultant movement of slider I0 in network causes operation of amplifier relay 59. The energization of relay 59 completes the circuit to energize servomotor winding I9 to position the elevator control surface in a downward direction tending to increase the downward acceleration above that provided by the down draft alone. However, the accelerometer coil I I5 is energized along with the servomotor winding I9 and reduces the threshold of acceleration at which switch contacts I00 and I01 are separated to operatively disengage the servomotor from the elevator control surface.

While the operation of the accelerometer coils H5 and H0 was considered relative to normal automatic pilot operation while the craft was subject to a vertical acceleration from an external disturbance, the operation may also be considered where malfunction of the automatic pilot requires a decrease in the threshold of the acceleration despite the absence of an up or down draft on the craft.

Assume that there has been a downward tilt of the craft about its pitch axis due to an external gust or large shift in the position of the center of gravity of the aircraft. The vertical gyroscope 82 would position slider I8 to unbalance network 00 to cause the amplifier 52 to close relay 58. With the closing of amplifier relay 58 the winding I8 of the servomotor is energized. It is assumed that the unbalance of network 60 is so large that the input signal tends to maintain relay 58 closed continuously so that coil I I6 is continuously energized over a sufficient period to make it effective.

If there were no rebalance of the network 50 due to a malfunction of one sort or another, the elevator surface would be continuously operated in an upward direction. This continuous upward movement of the elevator surface continues to increase the angle of attack. The continuous increase 'in angle of attack increases the lift of the craft and therefore increases the vertical acceleration of the craft.

By energizing coil IIG along with servomotor winding I'8, the threshold of acceleration to which weight I05 responds in such manner as to separate contacts I08 and I09 is reduced. Therefore the angular momentum of the aircraft about its Ipitch axis due to the displaced elevator surface after servomotor disengagement will not increase the acceleration of the craft above the magnitude at which the separation of contacts I08 and I09 occurred to such an extent that the acceleration ultimately attained is of such magnitude as to overstress the aircraft.

In a similar manner if due to a malfunction, the elevator surface is positioned in a downward direction the winding I I5 is energized along with the servomotor winding I9 to reduce the threshold of acceleration for separating contacts I06 and I01 to preventa downward acceleration being attained that would cause overstress of the craft structure.

It will now be evident that I have provided a safety device for monitoring the accelerations of an aircraft provided with an automatic pilot or steering mechanism which functions to disengage the automatic pilot from a steering surface of the craft on the attainment by the craft of vertical accelerations of predetermined magnitude in either of two directions and which further functions when the automatic pilot is not operating properly to reduce automatically the magnitude of the acceleration at which the disengagement occurs.

I claim as my invention:

1. In a steering mechanism for a dirigible craft having a control surface for changing craft attitude about an axis, a servomotor, control means for operatively engaging said servomotor and control surface, an accelerometer responsive to vertical acceleration of said craft, means operated by said accelerometer in response to a predetermined magnitude of acceleration for causing said control means to disengage said servomotor and control surface, means for energizing said servomotor, and means in said accelerometer operated during energization of said servomotor for decreasing the magnitude of acceleration at which disengagement of said servomotor and control surface occurs.

2. In a steering mechanism for a dirigible craft having a control surface for changing craft attitude about an axis, a servomotor, control means for operatively engaging said servomotor and control surface, an accelerometer responsive to vertical acceleration of said aircraft in both directions, means operated by said accelerometer in response to a predetermined magnitude of acceleration of said craft in either direction for causing said control means to disengage said servomotor and control surface, means for energizing said servomotor, and means in said accelerometer operated during energization of said servomotor for decreasing the magnitude of acceleration in one direction of said craft at which disengagement of said servomotor and surface occurs.

3. In a dirigible craft having a control surface which may be manually or power operated for changing craft attitude about an axis, a servomotor, control means for operatively engaging said servomotor and control surface, an accelerometer responsive to vertical acceleration, means operated by said accelerometer in response to a predetermined magnitude of acceleration of said craft for causing said control means to disengage said servomotor and control surface, means including a balanceable network for energizing said servomotor to maintain a predetermined attitude of said craft about said axis, and means in said accelerometer operated during energization of said servomotor for decreasing the magnitude of acceleration at which disengagement of said servomotor and surface occurs.

4. In a steering mechanism for a dirigible craft having a control surface for changing craft attitude about an axis, a servomotor, control means for operatively engaging said servomotor and control surface, an accelerometer responsive to vertical acceleration of said craft, means operated by said accelerometer in response to a predetermined magnitude of acceleration causing said control means to disengage said servomotor and control surface, adjusting means to vary the response of said accelerometer to change the value of acceleration at which said disengagement occurs, means for energizing said servomotor, and means in said accelerometer operated during energization of said servomotor 10 for automatically decreasing the magnitude of acceleration at which disengagement of said servomotor and control surface occurs.

5. In a steering mechanism for a dirigible craft having a control surface for changing craft attitude about an axis and including a servomotor and control means for operatively engaging said servomotor and control surface, an accelerometer responsive to linear acceleration of said craft said accelerometer including a magnetic weight and coacting contacts said weight upon the attainment of a predetermined linear acceleration causing separation of said contacts for causing said control means to disengage said servomotor and control surface, a coil for providing a magnetic field exerting an effect on said weight tending to separate said contacts, and means for energizing said coil during the positioning of said surface by said servomotor.

6. In a flight control apparatus for a dirigible craft, an accelerometer having a movable element responsive to vertical accelerations, means connected to said movable element for adjusting the acceleration at which said element moves, a servomotor, means controlled by said accelerometer and effective upon movement thereof for operatively disengaging said servomotor with a control surface of said craft, mean for energizing said servomotor,-and means acting on said movable element in a direction opposite to that of said adjusting means during the energization of said servomotor for lowering the magnitude of acceleration at which disengagement of said servomotor and said surface occurs.

'7. In a steering mechanism for a dirigible craft having a control surface for changing craft attitude about an axis, a servomotor, control means for operatively engaging said servomotor and control surface, a device responsive to a change in a condition of said craft, means operated by said device in response to a predetermined magnitude of change in said condition for causing said control means to disengage said servomotor and control surface, means for energizing said servomotor, and means in said device operated during energization of said servomotor for decreasing the magnitude of change of said condition at which disengagement of said servomotor and said surface occurs.

8. In a flight control apparatus for a dirigible craft having a control surface, means for positioning said control surface, an accelerometer having two relatively movable elements which on relative movement effects disengagement of said operating mean from said control surface, and means operated during continuous displacement of said control surface in one direction for lowering the magnitude of acceleration at which said relative movement occurs.

9. In an aircraft having a control surface for moving the craft about an axis, an automatic pilot having a servomotor adapted to be operatively connected to said control surface, means responsive to a predetermined magniture of vertical acceleration of said craft due to an exterior disturbance for severing operation of said control surface by said servomotor, and means associated with said responsive mean during positioning of said surface in a direction tending to increase said acceleration for decreasing the magnitude of the acceleration at which the severing of said servomotor and control surface occurs.

10. In control apparatus for an aircraft having a control surface for controlling the attitude of the. craft, a first accelerometer means responwe to vertical craft axis accelerations in one direction, a second accelerometer means responsive to vertical craft axis accelerations in an opposite direction, a servomotor for controlling the positioning of said surface, means for severing control of said surface from said servomotor and operated by said accelerometers upon a predetermined acceleration along said axes in either direction, and means for varying the response of an accelerometer in accordance with a continuously changing surface position, to reduce the magnitude of the acceleration at which said control is severed.

11. In an aircraft having control surface for moving the craft about an axis, an automatic pilot having a servomotor adapted to be operatively connected to said control surface, means responsive to a predetermined magnitude of a condition of said craft for severing operation of said control surface by said servomotor, and means associated with said responsive means during positioning of said surface in a direction tending to increase said condition for decreasing the magnitude of the condition at which the severing of said servomotor and control surface occurs.

12. In a dirigible craft having a control surface which may be manually or power operated for changing craft attitude about an axis, in combination: a servomotor; control means for operatively engaging said servomotor and control surface; an, amplifier having alternatively operable relays, for reversibly controlling said servomotor; a balanceable network connected to the amplifier for operating one or the other of said amplifier relays depending on the direction of unbalance of said network; an accelerometer responsive to vertical acceleration of the craft; means operated by said accelerometer for causing said control means to disengage said servomotor and control surface; and circuit means connected with said accelerometer and operated during energization of said servomotor during closing of an amplifier relay for increasing the effectiveness of said accelerometer.

ROBERT J. KUTZLER.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,461,533 Dose Feb. 15, 1949 2,487,793 Esval et a1. "Nov. 15, 1949 2,553,560 Esval May 22, 1951 2,604,613 Klass July 22, 1952 2,627,384 Esval Feb. 3, 1953 

